Bunuel it should be "use", not "sue" - please correct
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Firstly, note that Commercial Airlines != Private planes.

There are two entities here: Commercial airlines and private planes (furthermore, within private planes, there are two categories: ones with radar & ones without radar).



- No Impact.
What "commercial airlines" are required by law has no impact on the conclusion that when we enforce the policy that "private planes" with radars are only allowed ->leading to decrease in "private plane" traffic -> leading to decrease in risk of mid-air collision near central airports.



- more of a weakener.
If "commercial-airline" traffic is the main cause of the overcrowding (and therefore probably the risk of mid-air collision near central airports) then policy on "private planes" -> leading to reduction in "private plane" traffic has little impact on reducing risk of mid-air collision near central airports.



- weakener.
If private planes without radars are converted to with radars after the policy enforcement , then the traffic would continue at central airports and therefore risk of mid-collision near central airports won't reduce.



- No impact.
Sure. It can decrease. But it doesn't strengthen / weaken our belief that reducing "private plane" traffic -> will reduce risk of mid-air collision near central airports



- Strengthener.
If "Private planes" without radars are causing substantial mid-air collision, then reducing their traffic near central airports may reduce the risk of mid-air collision near central airports.

E. Since thats the only option relevant to the conclusion

The answer is E.
We can solve this question using the

LOGICAL

approach, since strengthening the conclusion is often a matter of identifying hidden assumptions.

The argument is that reducing the number of private planes without radars will decrease traffic accidents. The clear assumption here is that these types of planes are involved in a large part of accidents. This is exactly what E states.
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Thank you, David. Edited.
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If the airspace around centrally located airports were restricted to commercial airliners and only those private planes equipped with radar, most of the private-plane traffic would be forced to use outlying airfields. Such a reduction in the amount of private-plane traffic would reduce the risk of midair collision around the centrally located airports.

Conclusion: Reduction in private plane traffic will reduce risk of mid air collision
Pre thinking: 1. any assumption which says that why private plane traffic reduction will help in reducing collision will be strengthener

lets look at answer choices

Which of the following, if true, would most strengthen the conclusion drawn in the second sentence?

A. Commercial airliners are already required by law to be equipped with extremely sophisticated radar systems.

- Are we concerned about if airliners are required to be equipped with extremely sophsticated radar system, really no
irrelevant
B. Centrally located airports are experiencing overcrowded airspace primarily because of sharp increases in commercial-airline traffic.

- restating premise, we already know
C. Many pilots of private planes would rather buy radar equipment than be excluded from centrally located airports.
okay, but it do not explain why reduction in private planes will help in reducing collison
D. The number of midair collisions that occur near centrally located airports has decreased in recent years.

- irrelevant
E. Private planes not equipped with radar systems cause a disproportionately large number of midair collisions around centrally located airports.- this fill the gap and matches our prethinking


hence correct answer :E